The present invention is related to the field of electric wireline well logging tools. More specifically, the present invention is related to systems for communicating signals from logging tools disposed in wellbores to a recording system located at the earth's surface.
Electric wireline well logging tools are used to make measurements of certain properties of earth formations penetrated by wellbores. The measurements can assist the wellbore operator in determining the presence, and quantity if present, of oil and gas within subterranean reservoirs located within the earth formations.
Well logging tools known in the art are typically extended into the wellbore at one end of an armored electrical cable. The cable can includes at least one, and commonly includes as many as seven, insulated electrical conductors surrounded by steel armor wires. The armor wires are included to provide abrasion resistance and tensile strength to the cable. The cable supplies electrical power to the logging tools and provides a communication channel for signals sent between the logging tools and a recording system usually located near the wellbore at the earth's surface.
Logging tools known in the art can provide many different types of measurements of the earth formation properties, including measurements of electrical resistivity, natural gamma-ray radiation intensity, bulk density, hydrogen nucleus concentration and acoustic travel time, among others. Still other logging tools, generally called "imaging" tools, provide finely detailed measurements, meaning successive measurements can be made at axial and radial spacings of as little as several hundredths of an inch, of resistivity and acoustic pulse-echo travel time in order to generate a graphic representation of the visual appearance of the wall of the wellbore.
It is known in the art to digitize the measurements, which is to convert the measurements made by each one of the logging tools into binary digital "words" composed of a plurality of digital bits, bits being signal levels representing numerical "ones" and "zeroes". Digital words can represent the numerical values of the measurements sampled at spaced apart time intervals. The measurements are then typically transmitted to the recording system as a series of digital bits arranged in a predetermined pattern. The significance of the predetermined pattern will be further explained.
It is generally beneficial to the wellbore operator to be able to combine as many different types of logging tools as is practical into one continuous instrument package (generally called a "tool string" by those skilled in the art). The benefit to the operator is to reduce the number of times logging tools must be extended into the wellbore, which can save a considerable amount of operating time. Combining a large number of measurements generally requires that large amounts of signal data be sent to the recording system at the earth's surface.
A particular problem in combining large numbers of measurements in the tool string is that the large amount of signal data which must be transmitted can cause the required signal data transmission rates to exceed the signal carrying capacity of the cable. This problem is particularly acute when the imaging tools are included in the tool string because of the very fine measurement spacing, and consequently the large increase in the amount of signal data, of imaging tools relative to other types of tools.
The cable may have limited signal transmission capacity because of the need for the cable not to exceed a practical external diameter, the need generally being for reasons related to safety of the wellbore and personnel near the wellbore. A practical limit for the diameter of seven conductor cables known in the art can be about 19/32 of an inch. Limited cable diameter provides cables generally having electrical properties which limit their frequency response to less than about 200,000 Hz (200 kHz). Frequency response can be observed in FIG. 1, which is a graphic representation of the frequency response (also referred to as a "transfer function") of a typical well logging cable.
It is known in the art to increase the effective signal data carrying capacity of the logging cable by encoding the binary digital signals using various types of encoding methods. The encoding methods constitute the previously described "predetermined pattern" of digital bits. Quadrature amplitude modulation ("QAM") telemetry can be used to transmit well logging data to the recording system, and one type of QAM telemetry system used for well logging tools is described, for example, in U.S. Pat. No. 5,387,907 issued to Gardner et al. QAM telemetry includes conversion of groups of digital bits ("bit groups") in the previously described predetermined pattern into 2-dimensional symbols, each comprising coordinates corresponding to the bit values in each bit group. The coordinates are converted to in-phase and quadrature analog signal amplitudes which are used to drive a specialized analog signal modulator. The modulator controls the output amplitude of a signal carrier generator. The modulated carrier is applied to the logging cable. Signal data are recovered in the recording system by extracting the amplitude values from the modulated carrier and reconverting them to digital bits.
A drawback to QAM when used in wireline well logging tool signal telemetry is that precise recovery of the data signal impressed onto the carrier requires a complex and expensive signal demodulator to precisely recover the amplitude and phase of the carrier. It can be impractical to provide such a demodulator for use in wireline recording systems.
It is known in the art to provide a data transmission scheme using a modulation system which does not employ a signal carrier. Such a modulation scheme is therefore not subject to performance degradation by error in carrier recovery, nor does it require a complex and expensive demodulator. A so-called carrierless amplitude and phase modulation ("CAP") system is described, for example, in "Contribution T1E1.4/90-154, Carrierless AM/PM", by Sobrara et al, as presented to American National Standards Institute (ANSI) T1E1.4 Technical Subcommittee Working Group, 1990. A drawback to the CAP system as described in the Sobrara et al reference is that the system was designed for use on the public telephone network, rather than for well logging applications. The system described in the Sobrara et al reference can have inadequate ability to compensate for changes in signal timing that can particularly occur in wireline signal telemetry because of changes in electrical signal transmission properties of the cable and in signal timing generator changes in the well logging tools. Timing generator changes in the tools and electrical property changes in the cable can result from variation in the ambient temperature of the cable and the electronics in the well logging tools, as they are extended into and withdrawn from the wellbore.
Accordingly, it is an object of the present invention to provide a CAP telemetry system for use in wireline well logging which can compensate for timing generator changes and electrical transmission property changes in the logging cable.